Since most chemical carcinogens are also mutagens, assays of mutagenesis are useful as screening tests for chemical carcinogens. A mutagenesis assay has now been developed in which promutagens (procarcinogens) are metabolically activated by explants of human target tissues, e.g., bronchus. Mutation frequency (resistance to either 8-azaguanine or ouabain) is measured in mammalian cell line, V-79 Chinese hamster cells, which do not metabolically activate polynuclear aromatic hydrocarbons into mutagens. When human bronchial explants are cocultivated with V-79 cells in medium containing either benzo(a)pyrene (BP) or its proximate carcinogen, 7,8-diol, a marked increase in mutation frequency is observed. This increase is dependent upon dose of either BP or 7,8-diol, duration of cocultivation, and amount of human bronchus. At equimolar concentrations, 7,8-diol was activated to at least a 5-fold more potent mutagen than its parent compound BP. This human tissue-mediated mutagenesis test has potential as a: 1) screening test for chemical carcinogens; and 2) biological assay to be used with biochemical assays in an effort to determine oncogenic susceptibility among individuals as well as among different target tissues in a single individual.